Method of separating light emitting devices formed on a substrate wafer

ABSTRACT

A method according to embodiments of the invention includes growing on a first surface of a sapphire substrate a semiconductor structure including a light emitting layer disposed between an n-type region and a p-type region. The semiconductor structure is formed into a plurality of LEDs. Cracks are formed in the sapphire substrate. The cracks extend from the first surface of the sapphire substrate and do not penetrate an entire thickness of the sapphire substrate. After forming cracks in the sapphire substrate, the sapphire substrate is thinned from a second surface of the sapphire substrate. The second surface is opposite the first surface.

FIELD OF THE INVENTION

The present invention relates to methods for separating light emittingdevices grown on a substrate wafer.

BACKGROUND

Semiconductor light-emitting devices including light emitting diodes(LEDs), resonant cavity light emitting diodes (RCLEDs), vertical cavitylaser diodes (VCSELs), and edge emitting lasers are among the mostefficient light sources currently available. Materials systems currentlyof interest in the manufacture of high-brightness light emitting devicescapable of operation across the visible spectrum include Group III-Vsemiconductors, particularly binary, ternary, and quaternary alloys ofgallium, aluminum, indium, and nitrogen, also referred to as III-nitridematerials. Typically, III-nitride light emitting devices are fabricatedby epitaxially growing a stack of semiconductor layers of differentcompositions and dopant concentrations on a sapphire, silicon carbide,III-nitride, or other suitable substrate by metal-organic chemical vapordeposition (MOCVD), molecular beam epitaxy (MBE), or other epitaxialtechniques. The stack often includes one or more n-type layers dopedwith, for example, Si, formed over the substrate, one or more lightemitting layers in an active region formed over the n-type layer orlayers, and one or more p-type layers doped with, for example, Mg,formed over the active region. Electrical contacts are formed on the n-and p-type regions.

In some LEDs the growth substrate remains part of the final devicestructure, for example to provide mechanical stability to thesemiconductor structure. A significant amount of light may be emittedthrough the sides of the growth substrate. Side light emission from thesubstrate is undesirable in applications that require or prefer thatmost or all of the light be emitted from the top of the device.

US 2010/0267219 describes a method of thinning the growth substrate.According to the abstract, the method includes “a sapphire substrategrinding step of grinding the back side of the sapphire substrate; amodified layer forming step of applying a laser beam to the sapphiresubstrate from the back side thereof to thereby form a modified layer inthe sapphire substrate along each street; . . . and a wafer dividingstep of breaking the sapphire substrate along each street where themodified layer is formed”.

SUMMARY

It is an object of the invention to provide a method of separating lightemitting devices grown on a substrate by forming notches in thesubstrate, then thinning the substrate to expose the notches.

A method according to embodiments of the invention includes providing alight emitting semiconductor structure grown on a substrate. Thesubstrate has a front side and a back side opposite the front side.Notches are formed in the substrate. The notches extend from the frontside of the substrate into the substrate. After forming notches in thesubstrate, the back side of the substrate is thinned to expose thenotches.

A method according to embodiments of the invention includes growing on afirst surface of a sapphire substrate a semiconductor structureincluding a light emitting layer disposed between an n-type region and ap-type region. The semiconductor structure is formed into a plurality ofLEDs. Cracks are formed in the sapphire substrate. The cracks extendfrom the first surface of the sapphire substrate and do not penetrate anentire thickness of the sapphire substrate. After forming cracks in thesapphire substrate, the sapphire substrate is thinned from a secondsurface of the sapphire substrate. The second surface is opposite thefirst surface.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates one example of a III-nitride LED.

FIG. 2 illustrates a portion of a wafer of LEDs formed on a substrate.

FIG. 3 illustrates the structure of FIG. 2 after attaching the wafer tohandling tape and thinning the substrate.

FIG. 4 illustrates the structure of FIG. 3 after scribing the substrate.

FIG. 5 illustrates the structure of FIG. 4 after thinning the substrate.

FIG. 6 illustrates the structure of FIG. 5 after stretching the handlingtape to separate the LEDs.

FIG. 7 illustrates a mask applied to a portion of a substrate.

FIG. 8 illustrates the structure of FIG. 7 after etching notches in thesubstrate.

FIG. 9 illustrates the structure of FIG. 8 after stripping the mask.

FIG. 10 illustrates the structure of FIG. 9 after forming LEDs on thenotched substrate.

FIG. 11 illustrates the structure of FIG. 10 after attaching the waferto handling tape and thinning the substrate to separate the LEDs.

FIG. 12 illustrates a portion of a substrate including partially formedLEDs.

FIG. 13 illustrates the structure of FIG. 12 after forming notches inthe substrate.

FIG. 14 illustrates the structure of FIG. 13 after finishing the LEDs.

FIG. 15 illustrates the structure of FIG. 14 after attaching the waferto handling tape and thinning the substrate to separate the LEDs.

FIG. 16 illustrates a portion of a wafer of LEDs formed on a substrate,with cracks formed between neighboring LEDs.

FIG. 17 illustrates the structure of FIG. 16 after thinning thesubstrate.

DETAILED DESCRIPTION

In embodiments of the invention, a sapphire or other growth substrateremains part of the final device structure, but is thinned to reduce oreliminate light emission from the sides of the growth substrate. Inembodiments of the invention, the wafer is first partially separated byforming separation zones, which are often notches or cracks in thesubstrate, through at least part of the thickness of the substrate. Thewafer is then fully separated by thinning the substrate until theseparation zones are reached. Embodiments of the invention areparticularly suited to applications that require all or a significantportion of light to be emitted from the top surface of a device, such assome automotive applications.

Though in the examples below the semiconductor light emitting devicesare III-nitride LEDs that emit blue or UV light, semiconductor lightemitting devices besides LEDs such as laser diodes and semiconductorlight emitting devices made from other materials systems such as otherIII-V materials, III-phosphide, III-arsenide, II-VI materials, ZnO, orSi-based materials may be used.

FIG. 1 illustrates a single III-nitride LED 12 that may be used inembodiments of the present invention. Any suitable semiconductor lightemitting device may be used and embodiments of the invention are notlimited to the device illustrated in FIG. 1. The device of FIG. 1 isformed by growing a III-nitride semiconductor structure on a portion ofa growth substrate 10 as is known in the art. The growth substrate isoften sapphire but may be any suitable substrate such as, for example,SiC, Si, GaN, or a composite substrate. The semiconductor structureincludes a light emitting or active region sandwiched between n- andp-type regions. An n-type region 14 may be grown first and may includemultiple layers of different compositions and dopant concentrationincluding, for example, preparation layers such as buffer layers ornucleation layers, and/or layers designed to facilitate removal of thegrowth substrate, which may be n-type or not intentionally doped, and n-or even p-type device layers designed for particular optical, material,or electrical properties desirable for the light emitting region toefficiently emit light. A light emitting or active region 16 is grownover the n-type region 14. Examples of suitable light emitting regionsinclude a single thick or thin light emitting layer, or a multiplequantum well light emitting region including multiple thin or thicklight emitting layers separated by barrier layers. A p-type region 18may then be grown over the light emitting region 16. Like the n-typeregion 14, the p-type region 18 may include multiple layers of differentcomposition, thickness, and dopant concentration, including layers thatare not intentionally doped, or n-type layers.

After growth, a p-contact 20 is formed on the surface of the p-typeregion. The p-contact 20 often includes multiple conductive layers suchas a reflective metal and a guard metal which may prevent or reduceelectromigration of the reflective metal. The reflective metal is oftensilver but any suitable material or materials may be used. After formingthe p-contact 20, a portion of the p-contact 20, the p-type region 18,and the active region 16 is removed to expose a portion of the n-typeregion 14 on which an n-contact 22 is formed. The n- and p-contacts 22and 20 are electrically isolated from each other by a gap 25, shownhatched, which may be filled with a dielectric such as an oxide ofsilicon or any other suitable material. Multiple n-contact vias may beformed; the n- and p-contacts 22 and 20 are not limited to thearrangement illustrated in FIG. 1. The n- and p-contacts may beredistributed to form bond pads with a dielectric/metal stack, as isknown in the art.

In order to form electrical connections to the LED 12, one or moreinterconnects 26 and 28 are formed on or electrically connected to then- and p-contacts 22 and 20. Interconnect 26 is electrically connectedto n-contact 22 in FIG. 1. Interconnect 28 is electrically connected top-contact 20. Interconnects 26 and 28 are electrically isolated from then- and p-contacts 22 and 20 and from each other by dielectric layer 24,shown hatched, and gap 27. Interconnects 26 and 28 may be, for example,solder, stud bumps, gold layers, or any other suitable structure. Manyindividual LEDs 12 may be formed on a single wafer then diced from awafer of devices, as described below.

Though the embodiments below show separating a wafer into individualLEDs 12, the techniques described may be used to separate a wafer intogroups of LEDs. Though the embodiments below refer to a sapphire growthsubstrate, the techniques described may be applied to any suitablesubstrate.

One embodiment of the invention is illustrated in FIGS. 2, 3, 4, 5, and6. In the embodiment illustrated in FIGS. 2-6, the substrate 10 isthinned, then scribed, then thinned again.

In FIG. 2, an exemplary group of several LEDs 12 is formed on asubstrate 10. For example, LEDs 12 may be the devices illustrated inFIG. 1 or any other suitable device. Although six LEDs 12 are shown,there is no expressed limit to the number of LEDs that may be created ona single substrate, nor are these LEDS required to be in a group. TheLEDs in the figure are simply examples of a “some” LEDs on a portion ofa substrate 10 or a complete substrate 10.

In FIG. 3, wafer handling tape 34 is attached to LEDs 12. A portion 30of the thickness of the growth substrate 10 is removed by any suitabletechnique such as, for example, mechanical techniques such as grindingleaving a remaining portion 32. The substrate 10 is thinned to athickness which accommodates the scribing described in FIG. 4. Thethickness of the substrate 10 in FIG. 2 before thinning may be, forexample, at least 300 μm thick in some embodiments and no more than 1500μm thick in some embodiments, though the substrate may be thicker than1500 μm in some embodiments. The remaining portion 32 of substrate 10may be, for example, no more than 300 μm thick in some embodiments, nomore than 275 μm thick in some embodiments, and no more than 250 μmthick in some embodiments.

In FIG. 4, the regions 38 between individual LEDs 12 or groups of LEDs12 are scribed to form cracks or notches 40 in the remaining portion ofsubstrate 32. The cracks 40 are localized in a portion of the thicknessof the substrate 32 that is closest to LEDs 12. The cracks 40 do notfully penetrate the remaining portion of substrate 32. Cracks 40 may beformed by, for example, laser scribing, where a laser 36 is shinedthrough the substrate 32 in regions 38, or stealth dicing, where amodified layer in the substrate is formed by focusing a laser inside thesubstrate. For example, a femto-second laser with wavelengths between266 and 355 nm may be used for laser scribing and a laser withwavelengths between 800 and 1100 nm may be used for stealth dicing.

In FIG. 5, the remaining portion 32 of the sapphire substrate 10 is thenthinned to expose the tops of cracks 40 formed in FIG. 4. The substratemay be thinned by any suitable technique including mechanical techniquessuch as grinding. The thickness of the removed portion 42 of remainingportion 32 may be at least 100 μm thick in some embodiments and no morethan 200 μm thick in some embodiments. The cracked portion 44 remainingafter thinning may be, for example, no more than 60 μm thick in someembodiments, no more than 50 μm thick in some embodiments, and no morethan 40 μm thick in some embodiments. The cracked portion 44 remainingafter thinning is sufficiently thick in some embodiments to mechanicallysupport the semiconductor structure. After the thinning illustrated inFIG. 5, some or all of the cracks 40 extend through the entire remainingthickness of cracked portion 44. Preferably all of the cracks 40 extendthrough the entire remaining thickness of cracked portion 44.

In FIG. 6, tape 34 may be stretched to separate individual LEDs 12 orgroups of LEDs 12 in the gaps 46 where cracks 40 were formed. Each LED12 or group of LEDs 12 has a small piece of substrate 10 (crackedportion 44) attached to the top of the semiconductor structure. Thecracked portion 44 may be thick enough to mechanically support thesemiconductor structure. The cracked portions 44 may have smooth orrough edges.

Another embodiment is illustrated in FIGS. 7, 8, 9, 10, and 11. In theembodiment illustrated in FIGS. 7-11, the substrate is etched, thenthinned.

In FIG. 7, a mask 50 is formed over a sapphire substrate 10 andpatterned to form openings 52 aligned with areas where the substrate islater separated. The openings 52 may correspond to the edges of a singleLED or multiple LEDs in a group. The semiconductor structure may begrown on substrate 10 before or after forming openings 52. Thesemiconductor structure may be patterned such that the semiconductorstructure is removed from regions between LEDs.

In FIG. 8, the sapphire substrate 10 is etched to form notches 54 insubstrate 10 in the openings 52 in mask 50. In some embodiments, notches54 are at least one micron deep. In some embodiments, notches 54 are atleast one micron wide. The substrate 10 is etched by any suitabletechnique such as, for example, dry etching or wet etching.

In FIG. 9, the mask 50 is stripped, leaving substrate 10 with notches 54formed in regions 52.

As an alternative to the masking, etching, and stripping techniqueillustrates in FIGS. 7, 8, and 9, in some embodiments, before growingthe semiconductor structure of LEDs 12, notches are formed in thesubstrate 10 by a technique other than etching. For example, notches 54may be formed by laser scribing or stealth dicing, as described above,by laser dicing with a UV laser with a wavelength between 266 and 355nm, or by mechanical dicing, for example using a blade. Such techniquesmay not require first masking the substrate, though a patterned mask maybe used.

In FIG. 10, LEDs 12 are formed in the regions between notches 54. Forexample, the semiconductor structures grown on substrate 10 may beformed into LEDs by etching and forming metal layers to form contactsand interconnects, as described above in reference to FIG. 1.

In FIG. 11, the LEDs 12 are connected to wafer handling tape 34.Substrate 10 is thinned by any suitable technique, as described above.The portion 56 of the substrate 10 that is removed is sufficiently thickthat some or all of notches 54 are exposed, separating individual LEDs12 or groups of LEDs. Preferably all notches 54 are exposed. A portion50 of substrate 10 remains attached to each LED 12 or group of LEDs.Portion 50 is sufficiently thick in some embodiments to mechanicallysupport the semiconductor structure of LEDs 12.

Another embodiment is illustrated in FIGS. 12, 13, 14, and 15. In theembodiment illustrated in FIGS. 12-15, the LEDs are partially formed,the substrate is notched, the LEDs are completed, then the substrate isthinned.

In FIG. 12, LEDs are partially formed on substrate 10. For example, thesemiconductor structure of the LEDs may be grown on substrate 10.

In FIG. 13, notches 54 are formed in the substrate 10 in the regions 52between individual LEDs or between groups of LEDs. Notches 54 may beformed by any suitable technique, including, for example, etching,sawing, or laser scribing.

In FIG. 14, the LEDs 12 are completed for example by etching and formingmetal layers to form contacts and interconnects, as described above inreference to FIG. 1.

In FIG. 15, the LEDs 12 are connected to wafer handling tape 34.Substrate 10 is thinned by any suitable technique, as described above.The portion 56 of the substrate 10 that is removed is sufficiently thickthat some or all of notches 54 are exposed, separating individual LEDs12 or groups of LEDs. Preferably all notches 54 are exposed. A portion50 of substrate 10 remains attached to each LED 12 or group of LEDs.Portion 50 is sufficiently thick in some embodiments to mechanicallysupport the semiconductor structure of LEDs 12.

Another embodiment is illustrated in FIGS. 16 and 17. In FIG. 16, LEDs12 are grown on a substrate 10. For example, LEDs 12 may be the devicesillustrated in FIG. 1 or any other suitable device. Regions 38 betweenindividual LEDs 12 or groups of LEDs 12 are laser scribed from the sideof the substrate 10 on which LEDs 12 are formed. The scribing formscracks or notches 40 in a portion of the thickness of substrate 10. Thecracks 40 are localized in a portion of the thickness of the substratethat is closest to LEDs 12. Cracks 40 may be, for example, at least 30μm deep in some embodiments, and no more than 100 μm in someembodiments.

LEDs 12 is then mounted on a structure 34 such as a frame, supportwafer, or handling tape, as shown in FIG. 17. A portion 30 of thethickness of the growth substrate 10 is removed by any suitabletechnique such as, for example, mechanical techniques such as grinding.The thickness of the substrate 10 before thinning may be, for example,between 300 and 2000 μm. In some embodiments, the substrate is thinnedbeyond a thickness where some or all of the tops of cracks 40 (in theorientation illustrated in FIG. 17) are reached. Preferably all of thetops of cracks 40 are reached. The remaining portion 32 of substrate 10is sufficiently thick to mechanically support the semiconductorstructure of LEDs 12 in some embodiments.

Light emitting devices formed by the techniques described above may haveseveral advantages. Because some of the substrate remains attached tothe final device, the fragile semiconductor structure is supported bythe substrate, which may reduce the occurrence of failure due tocracking, may eliminate the need for expensive and complex thick metalinterconnects that support the semiconductor structure, and mayeliminate the need for underfill or other structures to support thesemiconductor structure. The device can be solder mounted. In addition,because the substrate is thinned, the amount of light that escapesthrough the sides of the substrate may be reduced as compared to adevice where the entire thickness of the substrate remains attached tothe semiconductor structure. Accordingly, devices formed by thetechniques described above may avoid or reduce an efficiency penaltytypically associated with devices where the substrate remains attachedto the semiconductor structure.

Having described the invention in detail, those skilled in the art willappreciate that, given the present disclosure, modifications may be madeto the invention without departing from the spirit of the inventiveconcept described herein. Therefore, it is not intended that the scopeof the invention be limited to the specific embodiments illustrated anddescribed.

The invention claimed is:
 1. A method comprising: growing on a firstsurface of a sapphire substrate a semiconductor structure comprising alight emitting layer disposed between an n-type region and a p-typeregion; forming the semiconductor structure into LEDs; thinning thesapphire substrate from a second surface of the sapphire substrate in afirst thinning process, the second surface being opposite the firstsurface; after thinning the sapphire substrate from the second surfacein the first thinning process, forming cracks in the sapphire substrate,the cracks extending from the first surface of the sapphire substrateinto the substrate without penetrating an entire thickness of thesapphire substrate; and after forming cracks in the sapphire substrate,thinning the sapphire substrate from the second surface of the sapphiresubstrate in a second thinning process.
 2. The method of claim 1,wherein forming the cracks comprises laser scribing.
 3. The method ofclaim 1, wherein forming the cracks comprises mechanically scribing. 4.The method of claim 1, wherein forming the cracks comprises laserscribing the cracks from the second surface.
 5. The method of claim 1,wherein thinning the sapphire substrate from the second surface of thesapphire substrate in the second thinning process comprises thinning thesapphire substrate to expose the cracks.
 6. The method of claim 1,wherein the cracks are formed between individual LEDs.
 7. The method ofclaim 1, wherein the cracks are formed between groups of LEDs.
 8. Themethod of claim 1, wherein the cracks are between 30 μm and 100 μm deep.9. The method of claim 1, wherein the substrate is no more than 300thick after the substrate is thinned in the first thinning process. 10.The method of claim 1, further comprising separating at least some ofthe LEDs from one another.
 11. A method comprising: at least partiallyforming a plurality of light-emitting semiconductor devices on a firstsurface of a substrate, each of the semiconductor devices including arespective active layer situated between a respective n-type layer and arespective p-type layer; thinning the substrate from a second surface ofthe substrate in a first thinning process, the second surface beingopposite the first surface; after thinning the substrate from the secondsurface in the first thinning process, forming cracks in the substrate,the cracks extending from the first surface of the substrate into thesubstrate without penetrating an entire thickness of the substrate;after forming cracks in the substrate, thinning the substrate from thesecond surface of the substrate in a second thinning process; andseparating at least some of the semiconductor devices from one another.12. The method of claim 11, wherein the cracks are formed by laserscribing.
 13. The method of claim 11, wherein forming the crackscomprises at least one of mechanically scribing and laser scribing. 14.The method of claim 11, wherein thinning the substrate from the secondsurface of the substrate in the second thinning process comprisesthinning the substrate to expose the cracks.